This invention relates to automated positioning systems and, more particularly, to automatic positioning servomechanisms responsive to a command signal for controllably moving a positionable element to one of various positions with negative feedback for maintaining the position until another command signal is provided. Specifically, the invention is directed to a method and apparatus for normalizing the velocity or speed at which the positionable element is moved by the servomechanism for attaining increased speed consistency and positional precision, especially in equipment characterized by a high frequency of position changes, such as printers, for example, daisy wheel printers, disk drives, and other equipment used in information processing.
By way of example, the invention will be described in connection with printers, namely, daisy wheel printers, in order to facilitate an understanding of the invention. The exemplary use of the invention in connection with a daisy wheel printer, however, is by way of illustration only and is not to be construed as a limitation of the principles of the invention to daisy wheel printers or to printers generally. As will become clear, the principles which underlie the invention have application to all automatic positioning servomechanisms for improving the precision with which a movable element is positioned by the servomechanism, especially under circumstances where the positionable element is indexed rapidly among positions.
Focusing on daisy wheel printers, a petal shaped print wheel is rotatably mounted to a carriage. The carriage is reciprocally mounted with respect to a paper feeder such that the reciprocal movement of the carriage is orthogonal with respect to the direction of paper feed, for example, the paper is fed vertically and the carriage is moved horizontally. Relative movement of the paper, the carriage, and the print wheel, and actuation of the print wheel, enables lines of characters to be printed. Printing requires that the paper be indexed to a line position, the carriage be moved to a character position, and the print wheel be rotated to a character, whereupon a hammer strikes the wheel causing the wheel to strike a ribbon interposed between the wheel and the paper, thereby imprinting a character on the paper.
The print wheel and the carriage are moved to the appropriate positions by respective servomechanisms. In the case of the print wheel, the servomechanism is under the control of a host which selects the characters to be printed. Desirably, the print wheel is rotatable in both a forward and a reverse direction in order to optimize the speed with which the character is advanced into position for printing, thereby increasing the throughput of the printer. Similarly, the order in which the characters are printed is determined by the host by control of the carriage position. Desirably, the carriage is also movable in a forward and a reverse direction to a character position in order to optimize throughput.
In the case of both the print wheel and the carriage, it is desirable that the forward and reverse speed, that is, the magnitude of the velocity, which is direction independent, be the same or at least within a predetermined tolerance and that the speed in both directions be within a certain range of a normalized speed. The normalized or nominal speed is the specified speed at which the printer is configured to operate and is dependent upon such factors as the physical dimensions and weight of the print wheel and carriage and the gain of the servomechanisms included in the printer, as well as factors determined by the host, such as space between characters. Importantly, and a long extant problem to which the invention is addressed, is that if the speeds of the respective print wheel and carriage do not satisfy this criterion, the quality of the print suffers. That is, if there is a disparity between the specified and actual speeds, positioning is imprecise, which results in poor registration of characters with respect to a line on the paper, only a portion of the character being printed, and other unacceptable deviant print characteristics.
Various factors cause the actual speed of the print wheel, as well as the carriage, to vary from the specified speed during movement. Several of the factors are inherent, such as tolerance variations throughout the manufacturing process in the production of the print wheel and carriage and other mechanical elements, as well as the electrical elements. Other factors are attributable to the environment in which the printer is deployed, such as dusty or dirty surroundings or installations in which the ambient temperature experiences seasonal changes such that the temperature during one day of operation is different from the temperature during another day of operation. These factors necessitate factory adjustment at the time of manufacture and/or precipitate periodic service calls when print quality deteriorates or the printer malfunctions.
In the past, the control circuit for the servomechanism has included at least one potentiometer which is adjusted for providing compensation in order to normalize the speed to the speed specified for the daisy wheel printer. An extender card is required in order to provide access to test points. An oscilloscope is viewed while the potentiometer is operated for monitoring the time for movement from one position to another so that the speed of movement results in proper positioning of the print wheel and carriage for printing in synchronism with the command signal from the host (i.e., the timing of the printing is synchronized).
The known manual adjustment using potentiometers has several disadvantages. Firstly, precision potentiometers are expensive. In addition, manual adjustments are difficult to perform and therefore require a skilled technician. Moreover, manual adjustments consume a significant amount of time and, therefore, are expensive. An initial adjustment at the factory is required, and frequent service calls are not unusual. Furthermore, if any parts of the printer are repaired or replaced or if circuit boards need to be interchanged, the need for adjustment is probable, which compounds the expense of repair or maintenance of the printer.
The invention provides automatic speed adjustment or compensation without the use of expensive precision potentiometers, an extender card, and an oscilloscope. The method and apparatus in accordance with the invention eliminate the need to fine tune a daisy wheel printer after manufacture for the purpose of adjusting for speed variations caused by tolerance variations throughout the manufacturing process and evidenced by differences in both the print wheel speed and the carriage speed, as well as differences in the speeds of the print wheel and carriage attributable to changes in environmental conditions. Moreover, the invention provides interchangeability of modular circuit boards among printers. Not only is factory adjustment eliminated, but frequent service calls are avoided. Furthermore, the average print quality between scheduled service calls for preventive maintenance is statistically improved; that is, the likelihood that print quality will deteriorate over the period between routinely scheduled service calls is substantially diminished.